The present invention relates to systems for detecting the passage of a mobile at a predetermined point on its guided displacement along a track, and it has a particularly important, although not limiting, application to automatically controlled public transport installations.
Detection systems are already known of the type that comprises an interrogation assembly and a passive responder, (i.e. having no electrical power supply of its own, neither by means of a battery or storage battery, nor by means of connection to a power supply network), one being carried by the mobile and the other carried by the track, in which:
the interrogation assembly includes firstly a low frequency transmitter and secondly a medium frequency transmitter designed to operate continuously, the transmitters having respective antennas transmitting towards a zone that is predetermined relative to the antennas and through which the responder passes during displacement of the mobile, and secondly a unit that is responsive to the characteristics of the responder when the latter is in the zone, said interrogation assembly being arranged to be connected to an electrical power supply; and
the responder comprises a circuit for receiving the medium frequency signal and a circuit for receiving the low frequency signal which is arranged to control the medium frequency circuit.
Among existing systems, particular mention may be made of those in which each responder comprises two members which, when a mobile passes, generate medium frequency signals and which are offset along the track so that the sum of signals received by the interrogation assembly is subjected to a sudden variation as a mobile passes a determined point of the zone.
In general, the passive responder is carried by the track, but it could be carried by the mobile; it is often designated by the term "marker" or "beacon". Since it does not need any electrical power supply and since it is cheap, it is possible to distribute a large number of responders along a track which are single-piece non-reparable units.